During well completion of a fully drilled oil and/or gas well, a number of casings of different lengths and diameters will be cemented to the ground formation. Between the casings, which are disposed coaxially to each other, a so-called annulus will be formed. To prevent a leakage in the oil and/or gas well, a plurality of packer elements will suitably be arranged between the casings. The casings will be suitably suspended from a wellhead structure, where the wellhead is arranged at the top of the oil and/or gas well. The present invention will be suitable for a surface well head on board a vessel or platform located at or near the water surface, but may also be applicable for sub sea use.
A wellhead of this kind is subjected to large loads and stresses from the surrounding environment. Although the well heads are designed to be maintenance-free for a number of years, it is both desirable and necessary to carry out an inspection not only during production, but also during drilling, installation and maintenance and repair work. This inspection may take place in the form of automated operations and is carried out by monitoring a number of different parameters in the well, which parameters may for example be contamination, leaks, well pressure, the production itself, sand/erosion in the well, wellhead temperature, the state or condition of various equipment (for example, the position of a valve), corrosion etc.
It is important from a safety, reliability and cost aspect to prevent a so-called pressure leak from the well through the different annuli in the casings, and to the surroundings. If an undesirable pressure leak of this kind nevertheless occurs, various safety systems are provided to close the well even under pressure, so that well fluid which has flowed into the different annuli of the well can circulate out in a controlled manner.
By carrying out constant measurements of, for example monitoring the pressure at a number of different points in the well, an indication that a pressure increase is about to occur in the well, that a pressure leak in the well will or has already occurred, may be present at an early stage and actions can be taken to ensure that the consequences of such a pressure build-up will be minimal or prevented altogether.
Various solutions have therefore been developed to monitor and/or control pressure in an oil or gas well, Reference can be made, for example, to U.S. Pat. No. 5,172,112, in which there is known that a pressure-measuring device measures pressure in a subsea pipe. The device includes a stationary unit mounted to the exterior of the subsea pipe and a movable unit that is lowered into position next to the stationary unit whenever the pressure is to be monitored or measured. The stationary unit, which is a strain gauge, will monitor the pressure in the pipe by measuring the “strain” in the pipe. The measurements will subsequently be transmitted from the stationary unit in the form of suitable signals, whereby the movable unit will then convert these signals to give a picture of the pressure that is within the subsea pipe.
A solution is known from GB 2 286 682 where an inductive pressure transducer is used to measure the pressure within a pipe. This is accomplished by passing an alternating current within an inductor coil to generate a magnetic field. The magnetic field passes through a gap formed between the pipe and the inductor coil, and then into the pipe. The fluid flowing in the pipe will, owing to its pressure, induce stress in the pipe, which stress will cause variations in the electromagnetic properties of the material from which the pipe is made, which variations can be sensed by the magnetic field that is formed. The sensed variations can then be converted to give a pressure measurement.
A feature common to the solutions described above is that the pressure-measuring device is not arranged through the material of the measuring element. This means that the pressure-measuring device can be replaced without any danger of a leakage or the like occurring during the replacement thereof, but on the other hand these known solutions will not give a satisfactory measurement as the measuring element may be affected by the ambient temperature, loads to which the measuring element is subjected etc.
Another system for detecting a leakage in an oil and/or gas well is described in U.S. Pat. No. 4,116,044, where the system comprises a plurality of pressure-sensitive transducers that are arranged in a through hole in a wellhead. The pressure-sensitive transducers will be so arranged that they can detect a leakage in a plurality of annuli in the well. The transducers are connected through wires to a junction box which will be capable of carrying signals to a processing location. During replacement of the transducers, the well will have to be shut down as the replacement operation will involve the well being “opened”.
WO 03/016673 addresses the inconvenience of shutting the well down to install or to replace the sensor. A tool for installing and removing a replaceable sensor in a subsea installation is described in WO 03/016673. The shown equipment comprises a sensor and the belonging equipment which is arranged at a rather vulnerable position at the outside of the well head